An implantable glucose sensor will be developed based on competitive binding between glucose and high molecular weight chromophoric or fluorescent ligands for lectins. The release of the fluorescent ligand will be monitored by single optical fibers. The miniaturized transducer, about 0.3 x 3 mm in size, will be placed at the end of a hyperdermic needle. Implantable sensors for other metabolites will be devised based on the same principle.